Opthalmodynamometry is a standard non-invasive technique for measuring correlates of intraocular pressure. A theoretical mechanical model will be developed to relate CSF pressure and modified optahlmodynamic pressure. The model will be tested by making opthalmodynometric measurement on patients who are already having CSF pressure monitored invasively. As necessary, the model will be adjusted, and it will be determined to what extent the technique is a useful clinical measure of CSF pressure. A modified opthalmodynamometer will be constructed to make these measurements.